As amplifiers of a high-frequency analog signal, amplifiers disclosed in PTL 1 and PTL 2 have been known up to now.
PTL 1 discloses, as illustrated in FIG. 21, an amplifier employing an ET system (envelope tracking) which changes a DC bias voltage on an output side of a high-frequency amplifier element according to an envelope signal of an input modulated signal as means for enhancing an efficiency of a high-frequency amplifier 1603. That is, as a method of making the output side DC bias voltage variable, a current controlled variable resistor (transistor) 1610 is inserted in series with a variable voltage switching power supply 1609 to realize the ET system. Reference numeral 1601 denotes an input terminal, 1602 is an output terminal, 1604 is a directional coupler, 1605 is a detector circuit, and 1606 is a peak detector. Also, the high-frequency amplifier disclosed in PTL 2 includes a voltage controlled circuit that changes a voltage to be applied to a drain electrode or a collector electrode of a semiconductor amplifier element substantially in proportion to an envelope component of an input signal which is supplied to a control electrode of the semiconductor amplifier element. The voltage controlled circuit has a plurality of batteries connected in series at connection points, and a control transistor is connected in parallel to the respective connection points.
Paragraphs [0066] to [0106] and FIG. 6 of PTL 3 disclose a power amplifying device of an envelope tracking four-voltage switching system in which four fixed voltage power supplies each having a different potential are connected in parallel to a supply terminal of a power amplifier through respective diodes, and power changeover switches 51 to 53 of three power circuits in the high-frequency amplifier turn on/off according to an envelope of the input signal to switch a supply voltage to be applied to the power amplifier in a stepped manner.
Paragraphs [0002] to [0006] and FIG. 5 of PTL 4 disclose an amplifier that switches a supply voltage according to a magnitude of an input signal in which a supply voltage switching circuit includes a plurality of switches which are connected in series with a pair of transistors configuring a push-pull circuit, and conducts on/off operation, and a plurality of supply voltages which are connected to interstages of the respective switches through respective diodes.
PTL 5 discloses an amplifier circuit that switches a supply voltage according to a signal, which includes a comparator that compares an output signal of an amplifier with a supply voltage, and controls the supply voltage switching circuit according to an output of the comparator.
PTL 6 discloses
A Power amplifier for multi-mode terminals that accepts a plurality of modulation systems in which an amplification control unit calculates a ratio of a peak power to a mean power for each modulation mode, and makes a supply voltage variable on the basis of the calculation results.
PTL 7 discloses a power amplifier circuit including an RF choke connected between a collector of an amplification transistor and a power supply, and a capacitor and a bias circuit between the collector and an output terminal.
PTL 8 discloses an amplifier of an ET (envelope tracking) system which controls a drain voltage of a power amplifier stage according to an envelope variation of a transmit signal in which the drain voltage is controlled according to a frequency Pr at which a ratio of an instantaneous power to a mean power of the transmit signal exceeds a given threshold value.